Recently, FRP, especially, CFRP (carbon fiber reinforced plastic) has got into the limelight as a structural member for the fuselage and wings of an airplane. This CFRP, in some cases, is provided as a composite plate member in which CFRP is combined with relatively tenacious metal such as aluminum or titanium.
The composite plate member is used in the form of a laminated plate in which metal is laminated on one surface of CFRP (which is hereinafter simply called a laminated plate). In this laminated plate, for example, when it constitutes the fuselage or wings of an airplane, it is necessary to drill a hole through which a fastening member such as a bolt or a rivet is inserted.
In this drilling, there is used a special drill proposed by the present applicant in the below-cited patent document 1 or an ordinary drill of a standard blade type.
The drill disclosed in the patent document 1 includes a cutting edge which is disposed in the front end of a body of the drill and extends from the rotation center of the drill to the outer periphery thereof. The cutting edge includes at least three portions, namely, a rotation center portion, an intermediate cutting edge portion connected to the outer end of the rotation center portion, and an outer-most peripheral cutting edge portion connected to the outer end of the intermediate cutting edge portion, while the point angles of the respective cutting edge portions reduce gradually from the rotation center side to the outer peripheral side cutting edge portion.